The problem is that in 5E, there is no 'backstab' (rogue sneak attacks are a different issue), so whatever this is, it's not in the 5E rules. Even flanking is only 1) an optional rule in 5E, and 2) requires a flanking opponent threatening the target. Giving auto-advantage when someone is behind you - when you allow no way to react (like an oppurtunity attack) or stop them moving there - is just overpowered and devalues advantage.
Just walk behind them - hey presto,advantage. Given that flanking is not even required for 5E, and is still more constrained than this, adding this arbitrary backstab rule is pretty overpowering, so everyone is going to use it. I certainly did, and so does the enemy AI. I just think they need to remove it altogether, or implement actual 5E flanking.